1. Field of the Invention
This invention relates to a Liquid Crystal Display (LCD), and more particularly to a LCD of the transflective type.
2. Description of the Related Art
LCDs can be classified based upon the source of illumination. Reflective displays are illuminated by ambient light that enters the display from the front. A reflective surface, such as an aluminum or silver reflector placed behind the LCD assembly, returns light to illuminate the LCD assembly while preserving the polarization orientation of the light incident on the reflective surface. Although reflective displays meet the need for low power consumption, the displays often appear rather dark and are therefore difficult to read. In addition, there are many conditions where there is insufficient ambient light, the purely reflective display is thus limited in usefulness.
In applications where the intensity of ambient light is insufficient for viewing, supplemental lighting, such as a backlight assembly, is used to illuminate the display. The typical backlight assembly includes an optical cavity and a lamp, LED or other structure that generates light. Although supplemental lighting can illuminate a display regardless of ambient lighting conditions, it is an expensive drain on battery life. Thus, the batteries on portable computers, for example, must typically be recharged after 2 to 4 hours of continuous backlight use.
In an attempt to overcome the above described drawbacks of reflective and transmissive displays, some electronic displays have been designed to use ambient light when available and backlighting only when necessary. This dual function of reflection and transmission leads to the designation, xe2x80x9ctransflectivexe2x80x9d. Transflective LCDs are a dual mode display device. These devices operate either with the available ambient light in a reflective mode or with an internal light source in the transmissive mode.
U.S. Pat. No. 6,211,992, issued on Apr. 3, 2001 to Van Aerle et al., discloses a prior transflective LCD with an electrode of chromium or aluminum that pass light in the transmissive state and reflect ambient light in the reflective state. However, to ensure that sufficient light can be passed in the transmissive state, the electrode must not be thick (in the case of aluminum, for example, thinner than 150 angstroms. It is very difficult to provide such electrodes with sufficient accuracy. Variations in thickness cause large variations in light transmission and, as a result, lead to non-uniform display in both the reflective state and the transmissive state. In the case of relatively large panels, the small thickness additionally influences the drive behavior because the sheet resistance becomes too high.
Therefore, it is a primary object of the present invention to provide a liquid crystal display with transflective electrodes of aluminum compound such as aluminum nitride which overcomes, or at least reduces the above-mentioned problems of the prior art.
According to a preferred embodiment of the present invention, the liquid crystal display comprises a top plate comprising a transparent electrode; a bottom plate bonded to the top plate, the bottom plate comprising transflective electrodes of aluminum nitride; a liquid crystal layer sandwiched between the top plate and the bottom plate; and a light source behind the bottom plate. The transflective electrodes reflect incident ambient light and transmit light emitted by the light source. In the liquid crystal display according to the present invention, the transflective electrodes of aluminum nitride replace conventional transparent pixel electrodes such that an image is generated by the transflective liquid crystal display when either the ambient light is incident on the surface of the top plate or when the light is generated by the light source.
In the transflective liquid crystal display according to the present invention, sufficient light from a light source (backlight) can pass through the transflective electrodes of aluminum nitride, while, on the other hand, the transflective electrode still has such a thickness (for example approximately 250 angstroms) that thickness variations caused by process variations do not influence the uniformity of the display. Further, the sheet resistance decreases considerably, thereby enhancing the drive behavior.